Leaves of stevia belonging to the family Asteraceae (Stevia rebaudiana) contain a secondary metabolite called steviol, a kind of diterpenoid, and steviol glycosides are used as calorie-less sweeteners in food industries because they are about 300 times sweeter than table sugar. Obesity has grown internationally as a serious social issue, and the demand for calorie-less sweeteners has been increasing day by day also in terms of health promotion and medical expense reduction. Although aspartame, an artificially synthesized amino acid derivative, and acesulfame potassium are now used as artificial sweeteners, naturally occurring calorie-less sweeteners like steviol glycosides are expected to be safer and more likely to gain public acceptance.
As a result of sugar modification, stevia glycoside is finally converted into a glycoside with four sugar molecules, which is called rebaudioside A (FIG. 1). Stevioside, a trisaccharide glycoside of steviol serving as a precursor of rebaudioside A, is the highest in quantity; and hence rebaudioside A and stevioside are main substances responsible for the sweetness of stevia. In addition to these, other glycosides which appear to be reaction intermediates and analogues with different types of sugars are known to be present.
The enzyme gene leading to biosynthesis of rebaudioside A has been isolated through expressed sequence tag (EST) analysis of stevia (Non-patent Documents 1 and 2, Patent Document 1). Steviol is generated when ent-kaurenoic acid, which is a precursor of the diterpenoid gibberellin serving as a plant hormone, is hydroxylated at the 13-position by the action of ent-kaurenoic acid 13-hydroxylase (EK13H), which is a cytochrome P450 enzyme (FIG. 2) (Non-patent Document 3, Patent Document 1). Steviol is first glycosylated (monoglucosylated) at the 13-position hydroxy group by the action of UGT85C2 to thereby generate steviolmonoside. Steviolmonoside is further glucosylated at the 2-position of the 13-position glucose to thereby generate a disaccharide glucoside of steviol, called steviolbioside, or is further glucosylated at the 19-position carboxyl group to thereby generate a diglucoside of steviol, called rubusoside. When the thus generated steviolbioside and rubusoside are further glucosylated, steviol glycosides including stevioside and rebaudioside A would be generated. Enzyme genes known to be involved in the generation of steviol glucosides are UGT74G1 and UGT76G1.
UGT74G1 is known to catalyze glucosylation at the 19-position of steviolmonoside (Non-patent Document 1). UGT74G1 also causes glucosylation of steviolbioside to thereby generate stevioside, a triglucoside of steviol. This stevioside is the highest in content in stevia leaves and is known to be about 250 to 300 times sweeter that table sugar. This stevioside is further glucosylated by the action of UGT76G1 to generate rebaudioside A, a tetraglucoside of steviol, which is considered to be the sweetest (350 to 450 times sweeter than table sugar) and to have a good quality of taste.
Steviol glycosides are reported to improve their quality of taste and sweetness levels, particularly upon addition of a branched sugar to glucose at the 13-position (Non-patent Document 4, Patent Document 2). Thus, glycosyltransferases catalyzing these reactions would be important enzymes responsible for determining the sweetness properties of stevia. 
Previous studies (Non-patent Document 2) have reported several types of glycosyltransferases (UGTs) as a result of EST analysis on stevia leaves, but detailed enzyme activity has not been fully examined for all of these enzymes.